1. Field of the Invention
The present invention relates to a laser power control technique required when recording and reproducing data in and from optical recording media, such as DVD disks, with a laser beam.
2. Description of Related Art
Along with wide use of multimedia, various types of read-only storage media (such as music CDs, CD-ROMs, and DVD-ROMs), as well as playback equipment, are in practical use. Recently, phase-change recording media have come to the front, in addition to recordable disks using dye-type media and rewritable MOs using magneto-optical (MO) media. Especially, rewritable DVDs are attracting a great deal of attention as the next-generation multimedia recording media and as large-capacity storage media.
In phase-change media of optical data storage, the phase of the recording layer changes in a reversible manner between the crystalline state and the amorphous state when recording and reproducing data. Unlike MO media, phase-change media do not require an external magnetic field. Data can be recorded and reproduced using only a laser beam emitted from a laser diode (LD), and overwrite recording can be performed by erasing recording data with a laser beam in one pass.
FIG. 1 is a timing chart showing an ordinary multipluse waveform used to record data in dye-type recording media. For dye-type media, a single pulse waveform is generated based on, for example, eight-to-sixteen modulation to record data on the dye-type recording layer. However, a single pulse recording method has such a problem that the recording mark written on the recording layer deforms into a tear-drop shape due to heat accumulation. To overcome this problem, it is proposed to produce a multipulse waveform of a laser beam based on EFM modulation, as an LD output waveform strategy, when forming marks on the recording layer. The 8 to 16 EFM signal and the multipulse waveform are indicated by symbols (b) and (c), resepectivcely, in FIG. 1.
As an example of the multipulse irradiation, it is proposed to produce a head pulse and subsequent pulses for heating the recording layer to define a mark.
FIG. 2 is a timing chart showing an ordinary multipulse waveform used to record data in phase-change recording media. Again, a laser beam with a multipulse waveform is produced to form a mark, but this waveform has multiple levels of recording power, as indicated by symbol (c).
For recording data in dye-type or phase-change recording media, it is necessary to correctly control the output power of the laser diode (LD). However, the driving current vs. output power characteristic of a laser diode easily fluctuates due to self-heating. In order to stabilize the output power, automatic power control (APC) is performed generally. In automatic power control, a portion of the laser beam emitted from the laser diode is guided onto a photo detector (PD), and laser driving current is controlled using a monitor current generated by the photo detector in proportion to the output power of the laser diode.
In general, a radio frequency electric current is superposed on the laser driving current to reduce noise. Considering only the data reproducing aspect, the APC can be realized easily by constructing a feedback loop of a relatively low frequency band because the laser driving current itself is a direct current.
On the other hand, when conducting the APC during the recording operation, laser power control becomes more difficult because the laser output power has to be changed at high speed to form marks and spaces. The recording power may be controlled to some extent with a simple structure as in data reproduction, by constructing a feedback loop of a low frequency band making use of the fact that the digital sum value (DSV) of the recorded data becomes zero in CD or DVD disks. However, the recording power cannot be controlled accurately.
To improve the control accuracy of the recording power, a sample and hold circuit may be used. For example, when data of the maximum length of 11T consisting of marks and spaces are recorded in a CD-R (dye-type) medium using the strategy indicated by symbol (c) in FIG. 3, the output power is sampled and held for each mark and each space. With this method, the recording power can be controlled more accurately at relatively low cost.
However, multipulse laser output is desired to record data in DVD disks of both dye-type and phase-change type, as has been explained above. For this reason, using a sample and hold circuit is unrealistic because high-frequency control is required for the light-receiving system and the subsequent stages.
To overcome this problem, JP 9-171631A proposes to appropriately insert a non-pulse driving period in the laser output waveform to control the output level of the laser between the amorphous level (at peak power) and the reading level (at bottom power) during the data recording operation in a phase-change medium.
However, if this technique is applied to the recording power control for mark formation in a dye-type medium based on the recording strategy shown in FIG. 1, a correct recording mark cannot be formed due to consecutive heating, and consequently, an area in which the recording operation is conducted during the non-pulse period results in a defective spot. This defective spot has little effect on the reproducing operation as long as error correction is carried out because automatic power control (APC) is implemented at a relatively long interval.
As to power control for space formation, the output power for forming a space is generally a constant power, and it can be controlled without causing defect in the recorded data by sampling and holding the output power at the timing of writing relatively long space data. This means that the space recording power can be controlled at a shorter interval as compared with the mark recording power.
Since the optimum level of the recording power varies depending on the surrounding temperature, as well as on type of the recording medium and the linear velocity, the recording power is optimized through test writing in the optimum power control (OPC) process. The OPC process is carried out by recording and reproducing prescribed test data on and from a power calibration area (PCA) in the recording medium.
To be more precise, test data of a prescribed pattern formed by marks and spaces with lengths of three times to fourteen times as long as the channel clock T (3T-14T) are recorded at several different levels of laser output power. Then, the test data are reproduced, and DC modulations of the RF signal and RF signal asymmetry after DC coupling are calculated as evaluation parameters at each level of output power.
The modulation M is calculated using equation (1), which is expressed by the ratio of maximum peak-to-peak amplitude Ip-p of the RF signal to the maximum level Imax of the RF signal.M=Ip-p/Imax  (1)
The asymmetry β after DC coupling is expressed by equation (2) using a positive peak level X1 and a negative peak level X2 of the RF signal after AC coupling.β=(X1+X2)/(X1−X2)  (2)where X1+X2 denotes the difference between the positive and negative peaks of the RF signal after AC coupling, and X1−X2 denotes the peak-to-peak value of the RF signal after AC coupling.
The optimum recording power is determined based on the modulation M and/or the asymmetry β after AC coupling.
To conduct actual recording operations, it is necessary to calculate the differential efficiency of the laser diode (LD) in advance and determine the driving current for each power level. In general, pre-recording is implemented in the focus-off state prior to the recording operation, and the recording output power levels are sampled to calculate the differential efficiency.
Conventionally, it has been considered that the driving current vs. laser output power characteristic (I-L characteristic) of a laser is substantially linear. However, to be more exact, the I-L characteristic exhibits a slightly a non-linear property. In addition, when outputting a laser pulse, the I-L characteristic varies in a short period of time due to abrupt change from low power to high power. As a result, the peak level of each pulse declines, as illustrated in FIG. 4. The peak drop becomes conspicuous as the output power becomes high. This is because the laser heats up more rapidly when the output power becomes higher.
In general, the higher the recording speed, the shorter the recording pulse. Accordingly, the quantity of peak drop increases when the recording speed becomes slower, as illustrated in FIG. 5A and FIG. 5B. The peak drop B of a long-width pulse shown in FIG. 5B is greater than the peak drop A of a short-width pulse shown in FIG. 5A (B>A).
If the laser driving current is determined on the assumption that that the I-L characteristic is linear, with little regard to the non-linear characteristic of the laser diode, output power error will occur and the recording quality will be degraded.
Another problem caused in conventional optical disk recording/reproducing apparatuses due to variation in peak drop phenomenon is deviation from the optimum full-scale value of the digital-to-analog converter.
Some kinds of optically recording and reproducing apparatuses employ a laser power control system that uses a digital-to-analog converter (DAC) as the current source for driving the peak power of the laser diode (LD), while digitally controlling the peak level. In such type of apparatus, another digital-to-analog converter is used to set the maximum driving current for the former DAC.
The latter DAC is called a full-scale DAC. The full-scale DAC adjusts the quantity of the maximum current required to drive the laser so as to optimize the resolution of the former DAC (for driving the peak level).
If a large amount of laser driving current is not so required, the value of the full-scale DAC is set small to increase the resolution of the peak-level driving DAC. For determining the full-scale DAC value, the differential efficiency of the laser has to be estimated in advance to determine how much driving current is required with respect to the maximum output power required under the given recording condition.
However, if the laser diode (LD) has such a characteristic that the peak drop becomes conspicuous along with increase of laser output power, the differential efficiency of the laser diode fluctuates depending on the output power. With such fluctuation, the optimum full-scale DAC value cannot be determined, and therefore, the resolution of the peak-level driving DAC cannot be set to the optimum.